In recent years, large-capacity external storage devices are desired as society becomes more and more computerized. In optical recording of information, there is conventionally a limit to higher density achieved by reducing the size of a recording pit because of a diffraction limit defined by the wavelength of light and the numerical aperture of an objective lens. In order to solve the above-mentioned problem, multilayer optical recording media having a plurality of recording layers are proposed (e.g., JP 5(1993)-151591 A). However, in such mutilayer-type optical recording media, a semitransparent film having a certain reflectance and transmittance with respect to light is used for the recording layers, which results in a light loss due to light reflection in recording layers other than a target recording layer. Moreover, with an upstream side in the traveling direction of incident light being the upper side, and a downstream side being the lower side, transmitted light also reaches layers located lower than the target recording layer, resulting in a further light loss.
In order to solve the above-mentioned problems, multilayer optical recording media that use a nonlinear material having nonlinear optical characteristics for recording layers are proposed (e.g., JP 2000-3529 A).
FIG. 11 shows a cross-sectional configuration of a conventional multilayer optical recording medium. The optical recording medium as shown in FIG. 11 includes a first recording layer 12 located between a first light transmission film 10 and a second light transmission film 14, and a second recording layer 16 formed so as to be opposed to the first recording layer 12 with the second light transmission film 14 sandwiched therebetween. Further, the first recording layer 12 is provided with guide grooves 12A. The first recording layer 12 is formed of a nonlinear reflective material having a reflectance that nonlinearly increases with the intensity of light. As the nonlinear reflective material having such properties, a-Si, InSb, ZnTe, ZnSe, CdSSe, GaAs, GaSb, or the like may be used. When the first recording layer 12 is formed of such a nonlinear reflective material, the first recording layer 12 has a reflectance changing in accordance with |(n−ns)/(n+ns)|2. Herein, “ns” represents the refractive index of the light transmission films 10 and 14, and “n” represents the refractive index of the first recording layer 12 as a nonlinear reflective material. The nonlinear reflective material used here is a material that causes a phenomenon in which the refractive index changes depending upon the light intensity, i.e., a material having a great nonlinear optical effect.
The optical characteristics of such an optical recording medium will be described. When the first recording layer 12 is accessed, a light spot is formed on the first recording layer 12, and accordingly the first recording layer 12 is irradiated with a relatively intense light beam. The reflectance of the first recording layer 12 at this time may be 40%, for example. On the other hand, when the second recording layer 16 is accessed, a radiated light spot is formed on the second recording layer 16, and accordingly the first recording layer 12 is irradiated with relatively weak light. The reflectance of the first recording layer 12 at this time may be 30%, for example, which means the first recording layer 12 reflects 30% of incident light and transmits 70% thereof to the second recording layer 16 side. Thus, the second recording layer 16 can be accessed efficiently.
However, the above-described prior art provides only a 10% reflectance change, from 30% to 40%. This is a limit due to the fact that the material having nonlinear optical characteristics is used for the first and second recording layers 12 and 16, and there is a problem that an amount of light would be insufficient for an optical recording medium that is further multilayered. Further, the above-described prior art relates to a read-only memory (ROM) in which information is recorded previously. Thus, it is difficult to ensure the energy required for recording information so as to realize a recordable multilayer optical recording medium with this art.